ABSTRACT Hookah (water pipe) smoking is a new global epidemic. The World Health Organization wants to prohibit all claims that hookah is less harmful than cigarettes and wants hookah products to bear the same warning labels as cigarettes. But there is little scientific evidence to substantiate this proposal. Hookah tobacco is heated with burning charcoal. So, the smoke contains ?tar? and nicotine plus charcoal combustion products including carbon monoxide (CO) and proatherogenic oxidants. With hookah, the CO boost decreases hemoglobin (Hb) O2-carrying capacity, which will decrease myocardial O2 delivery unless offset by increased myocardial blood flow (MBF). The goal of this revised R21 application is to test the novel hypothesis that: In young healthy hookah smokers, CO?a key charcoal combustion product in hookah smoke?acutely enhances nitric oxide (NO)/cGMP-mediated endothelial-dependent vasodilation in the coronary microcirculation, thereby masking acute tobacco-induced impairment. We further hypothesize that in long-term middle-aged hookah smokers: a) the coronary endothelium has become too dysfunctional to respond acutely to CO in hookah smoke, thereby unmasking acute tobacco-induced impairment in endothelial-dependent vasodilation; and b) the impaired microvascular perfusion plus reduced HbO2 carrying capacity from COHb plus increased myocardial work causes an O2 supply/demand imbalance causing subtle ischemic LV dysfunction detected by LV wall strain. Coronary endothelial function will be measured as the increase in myocardial blood flow (MBF) induced by isometric handgrip (increased systolic blood pressure and heart rate and thus shear stress). The MBF will be measured by perfusion imaging using quantitative myocardial contrast echocardiography. The handgrip- induced increase in MBF will be measured in younger and older hookah smokers before and after smoking charcoal-heated or electrically-heated hookah tobacco, and, for comparison, in age-matched cigarette smokers before and after smoking two cigarettes. Sixty subjects will be recruited to determine: 1) the acute effect of hookah smoking on coronary endothelial function in healthy young smokers 21-25 years of age; 2) the acute effect of inhaled CO gas alone (to mimic the safe hookah-induced CO boost of 25 ppm) on coronary endothelial function in the young hookah smokers; and, 3) the acute effect of hookah smoking on coronary endothelial function and left ventricular wall strain in long-term middle-aged smokers 35-49 years of age. The anticipated results of the proposed R21 stand to provide a novel mechanistic understanding of how hookah smoking impacts myocardial blood flow regulation, and, in so doing, inform new evidence-based public policy on hookah regulation.